The synthesis of diamond crystals by high pressure, high temperature processes has become well established commercially. Preferred methods for making diamonds are disclosed and claimed in U.S. Pat. Nos. 2,947,610 -- Hall et al and 2,947,609 -- Strong. Apparatus for the conduct of such processes is described and claimed in U.S. Pat. No. 2,941,248 -- Hall. The Hall et al, Strong and Hall patents are incorporated by reference.
Diamond growth in the aforementioned processes occurs by the diffusion of carbon through a thin metallic film of any of a series of specific catalyst-solvent materials. Although such processes are very successfully employed for the commercial production of industrial diamond, the ultimate crystal size of such diamond growth is limited by the fact that the carbon flux across the catalyst film is established by the solubility difference between graphite (the typical starting material) and the diamond being formed. This solubility difference is generally susceptible to significant decrease over any extended period due to a decrease in pressure in the system and/or poisioning effects in the graphite being converted.
On the other hand, in the method of growing diamond on a diamond seed crystal disclosed in U.S. Pat. No. 3,297,407 -- Wentorf, Jr. (incorporated by reference) a difference in temperature between the diamond seed and the source of carbon is relied upon to establish a concentration gradient in carbon for deposition on the seed. Catalyst-solvents disclosed in the aforementioned Hall et al and Strong patents are used in the temperature gradient method as well. The growth of diamond on the seed material is driven by the difference in solubility of diamond in the molten catalyst-solvent metal at the nutrient (source of carbon) and at the seed, between which locations a temperature gradient exists. Most important, this general type of reaction vessel configuration presents a pressure stable system so that pressure can more readily be kept in the diamond stable region.
By very carefully adjusting pressure and temperature and utilizing relatively small temperature gradients with extended (relative to growth times for thin film method) growth times, larger diamonds can be produced by the method as taught in the Wentorf patent than by the thin-film method.
Attempts to reliability produce very high quality diamond growth, however, have presented at least two apparently mutually exclusive, yet simultaneously occurring problems. These problems are:
a. the strong tendency for spontaneous nucleation of diamond crystals near the diamond seed material (which occurs with increase in the temperature gradient over the "safe" value); if the growth period is extended to produce from the seed diamond growth of greater than about 1/20 carat in size the nucleated growth competes with the growth proceeding from the diamond seed with subsequently occurring collisons of multiple crystals that result in stress fractures therein, and PA1 b. either partial or complete dissolution of the diamond seed material in the melted catalyst-solvent metal during that part of the process in which the catalyst-solvent medium becomes saturated with carbon from the nutrient source and then melts; such dissolution produces uncoordinated diamond growth proceeding from spaced loci, which growths upon meeting, result in subsequent confused, flaw-filled growth.